1. Field of the Invention
The present invention relates to an image signal coding system for coding analog image signals into digital signals in a high-definition digital video tape recorder(D-VTR).
2. Description of the Related Art
With the recent development in computers, semiconductor devices and digital signal processing techniques, developments have also occurred with respect to a conversion of analog image signals to digital signals and a storage and/or transmission technique for the digital converted signals.
In a case where analog image signals for a component television are converted into digital signals which in turn, are stored in a storage medium or transmitted through a signal transmitted channel, an amount of digital signal data to be processed is increased as compared with analog signal data.
When viewed, a limit in storage capacity of the storage medium and an effective practical use of the transmitting channel as well as a transmission of the image signal at a predetermined bit rate, a process of compressing the image signal is necessarily required.
In such an image compression, when the image signal data is reduced to a predetermined amount of digital data, an original signal information must be substantially expressed, and an error which is liable to be produced in the data storage medium has to have a steady characteristic. In addition, hardware for such an image signal compressing process must be easily constructed.
In accordance with a principle of such an image signal band compression, the image signals are subjected to a prediction coding or transform coding depending upon a correlation therebetween, and then quantized to produce the quantized factors. These factors can be produced by way of an information theory by which redundant data is minimized by using a statistical property of the image signal. The factors are then subjected to a variable length coding.
An image coding using a so-called discrete cosine transform has an advantage in that image signal energies are substantially concentrated in a lower frequency area to the digital-converted image signal and, hence, such image data can be sufficiently expressed by a small quantity of data when the lower frequency area is effectively coded.
The variable coding technique is defined such that a variable codeword is assigned to the given data to be converted on the basis of a probability density of the image data. The variable coding technique is preferably employed when the data to be processed lie to and for the image signal. Therefore, such image data unevenly distributed may be effectively expressed by way of the discrete cosine transform previously mentioned.
With such conventional coding methods which refer to only local character of the image to be processed, however, when the image signal is necessarily compressed to a target amount of data having a fixed length in, for example, the D-VTR, and the compressed image data is then stored in a storage medium or transmitted to a destination particularly when a composite image signal having variable portions and linear portion is presented therein, a redundancy generating characteristic is insufficiently considered and, thus, a serious deterioration in picture quality is likely to occur when the linear portions lie adjacent to the variable portions.
In a case where the image signal is subjected to the variable length coding, it is preferred that an amount of data per unit of time is substantially constant so as to store or transmit the coded image signal properly. For this reason, a buffer, which is controlled as a forward buffer control or backward buffer control, is employed with the image signal coding to adjust a quantization step size and, thus, to control the amount of the data to be produced during the image signal coding.
According to such a forward buffer control, an error in the storage medium is advantageously transmitted within a unit of a given fixed length, but it is difficult to precisely coincide the amount of data generated with a target amount of data.
In a case of the backward buffer control, a status of the buffer is checked for a constant period of time with regard to the quantization step size in accordance with a degree of fullness of the buffer, so that the amount of the data to be stored or transmitted can be preferably coincided with the target amount of data. However, if the variable portions of the image signal lie adjacent to the linear portions, a block effect is likely to occur due to an error which is produced in quantizing the image signal.
Further, when the D-VTR must transmit a target amount of data within a unit of limited fixed length to achieve special playback such as a high speed or fast playback, if the amount of the coded data produced exceeds a target amount of data, then it is necessary to employ hardware having a complex construction to remove such excessive factors.